In this paper I try to demonstrate how Byzantine law, a subject odd and exotic at first sight provides a piece of the puzzle that helps us to complete the big picture, the origins of our European legal identity. I refer to some concrete examples of legal interaction between the Byzantine and the Western side of Europe in the tenth, eleventh and twelfth centuries – a period in which the ius commune began to take shape – and explain the method I used step by step, the specific challenges I confronted in the sources and the outcomes of this approach. The comparative legal study of documents of the medieval period at a European level can help us to answer the question whether, long before the making of today’s Europe, today’s European countries were already connected by common legal forms.